1. Field of the Invention
The present invention relates to a non-impact printer and more particularly to a printer improved in printing quality to provide natural printing for a curve, a slash or the like.
2. Description of the Prior Art
Hitherto, in a non-impact printer such as an electrophotographic printer, an electrically charged photoconductor drum is illuminated with a light source to form an electrostatic latent image on a surface thereof, and developing is performed upon adhesion of a toner to the electrostatic latent image to form a toner image. Then, the thus obtained toner image is transferred to a recording material to be fixed thereon.
FIG. 2 is a block diagram of a printer control circuit in a conventional non-impact printer. In FIG. 2, a printing control unit 1 transmits, at the time a sheet arrives at a printing ready point, a timing signal 12, including a line and a raster timing signal, to a host or another controller, and receives a video signal 11 which has been edited on each page in the other controller. The video signal 11 received by the printing control unit 1 is transmitted to an LED (Light Emitting Diode) head 19 in the form of a real printing data signal 18.
FIG. 4 is a block diagram of the LED head in the conventional non-impact printer. In FIG. 4, the LED head 19 comprises a shift register 19a for storing in order a line of real printing data signal 18 from the printing control unit 1 shown in FIG. 2 in synchronism with a clock signal 18a, a latch 19b for temporarily holding a line of real printing data signal 18 stored in the shift register 19a in accordance with a latch signal 17, an LED group 19c wherein LED elements, the number of which corresponds to that of a line of dots, are arranged, and a driver group 19d for supplying the real printing data signal 18 held in the latch 19b to the LED group 19c.
The shift register 19a in the LED head 19 stores in order the real printing data signal 18 in synchronism with the clock signal 18a. Upon receipt of a line of video signal 11, the printing control unit 1 transmits a latch signal 17 to the LED head 19. The latch 19b holds a line of real printing data signal 18 stored in the shift register 19a in accordance with the latch signal 17. Before receiving the subsequent video signal 11 by the printing control unit 1, the thus held real printing data signal 18 is transmitted to the LED group 19c in accordance with a printing drive signal 13, so that the corresponding LED elements are lightened.
Transmission and receipt operation of the video signal 11 is performed in the unit of print lines. FIG. 3 is an operational time chart of the conventional non-impact printer mentioned above. However, according to the conventional non-impact printer mentioned above, the same size of dots are printed on a sheet depending on the resolution of the LED head 19 which is determined by the arrangement of the LED elements at regular intervals in a raster direction. Thus, there will be retained a serrated edge-like image on a slash portion of the printed image due to the resolution.
FIGS. 5A, 5B and 5C, and FIGS. 6A, 6B and 6C are views showing printing states according to the conventional non-impact printer. FIGS. 5A and 6A show 300 dots per inch (DPI) data; FIGS. 5B and 6B each show the relation between the print timing and driving energy for the LED head; and FIGS. 5C and 6C each show a real printing image.
As shown in the figures, both of the real printing images have undesirable serrated edge-like images, since the printing is performed on the predetermined raster lines. In view of the foregoing, if one contemplates increasing the density of the dots in order to improve the printing quality, then it is necessary to use an LED head 19 in which the LED elements are arranged at closer intervals or pitches. However, such an LED head 19 encounters a lower yield in manufacture of the parts where the LED elements are arranged, and thus becomes very expensive.